Corion actually answered the question, but only by guessing, so I will restate his answer but with some more details and certainty.

You have two choices here really, the first is a type union, which is written as Str | Undef and this basically says that the value can be either a String or Undefined. The second choice is the Maybe[`a] parameterized type, which would be written like this Maybe[Str]. This gives you basically the same thing as the type union but communicates the relationship differently by more explicitly saying "this *might* be undefined" not "this *can also* be undefined". My personal choice is always to do Maybe[`a] instead of the union in cases like these.